New and advanced transmission systems and braking systems are being developed by the automotive industry. These new systems often involve high-energy requirements. Therefore, the friction materials technology must be also developed to meet the increasing energy requirements of these advanced systems.
In particular, new high performance, durable friction materials are needed. The new friction material must be able to withstand high speeds wherein surface speeds are up to about 65 m/seconds. Also, the friction material must be able to withstand high facing lining pressures up to about 1500 psi. It is also important that the friction material be useful under limited lubrication conditions.
The friction material must be durable and have high heat resistance in order to be useful in the advanced transmission and braking systems. Not only must the friction material remain stable at high temperatures, it must also be able to rapidly dissipate the high heat that is being generated during operating conditions.
In particular, friction materials that are used in shifting clutches and the like are subjected to the gradual breakdown of the fluid in which the friction material is being used. The as the fluid is used over a long period of time the fluid may degrade and particles of such fluid degradation form. These particles remain in the fluid, causing a loss of coefficient of friction for the friction material.
The high speeds generated during engagement and disengagement of the new transmission and braking systems mean that a friction material must be able to maintain a relatively constant friction throughout the engagement. It is important that the frictional engagement be relatively constant over a wide range of speeds and temperatures. It is also important that the friction material have a desired torque curve shape so that during frictional engagement the friction material is noise or “squawk” free.
The principal performance concerns for all applications using friction materials are “hot spot” resistance and the energy management at the friction interface. The occurrence of “hot spots” can be attributed to many factors including the friction characteristics of the friction material, the mating surface's hardness and roughness, oil film retention, lubricant chemistry and interactions, clutch operating conditions, driveline assembly and hardware alignment, and driveline contamination. The friction interface energy management is primarily concerned with controlling interface temperature and is affected by the pump capacity, oil flow path and control strategy. The friction material surface design also contributes to the efficiency of interface energy management.
Previously, asbestos fibers were included in the friction material for temperature stability. Due to health and environmental problems, asbestos is no longer being used. More recent friction materials have attempted to overcome the absence of the asbestos in the friction material by modifying impregnating materials with phenolic or phenolic-modified resins. These friction materials, however, do not rapidly dissipate the high heat generated, and do not have the necessary heat resistance and satisfactory high coefficient of friction performance now needed for use in the high speed systems currently being developed.
The present invention is an improvement over the Seitz U.S. Pat. No. 5,083,650 reference which involves a multi-step impregnating and curing process; i.e., a paper impregnated with a coating composition, carbon particles are placed on the paper, the coating composition in the paper is partially cured, a second coating composition is applied to the partially cured paper, and finally, both coating compositions are cured.
The Fujimaki et al. U.S. Pat. No. 4,451,590 describes a friction material having metallic fibers, filler, carbon particles, carbon fibers and phenolic resin.
The Smith et al. U.S. Pat. No. 5,965,658 describes a friction material using of carbonaceous fibers derived from oxidized polyacrylonitrile based fibers and powdered metal and/or metal oxide friction materials.
The Winkler U.S. Pat. No. 5,662,993 describes a friction material having fibers bound along points of contact by a binder material.
The Irifune et al. U.S. Pat. No. 6,121,168 describes a friction material with fibers and porous cylindrical diatomaceous earth.
The Suzuki et al. U.S. Pat. No. 6,586,673 B1 describes the use of disc-shaped diatomaceous earth as a filler in a fibrous base material.
The Suzuki et al. U.S. Pat. No. 6,265,066 B1 describes a friction material comprising a fibrous base, a filler, a friction adjuster and a binder that contains a material having a siloxane bond.
The Suzuki et al. U.S. Pat. No. 6,231,977 B1 describes a friction material comprising a fibrous base material, a filler, a friction controller and phenolic resin binder which is modified with p-nonyl phenol. The Matsumoto et al. U.S. Pat. No. 6,060,536 describes a method of making a friction material where a water soluble material is dispersed into a raw paper which includes a fibrous base material, a filler, and a friction adjusting agent.
The Klink U.S. Pat. No. 4,522,290 describes a friction lining comprising a resin-free wood with an average raw density in the dried condition of at least 550 kilograms per cubic meter where the fibers of the wood run substantially parallel to the frictional direction of the clutch or brake.
The Jacko et al. U.S. Pat. No. 4,239,666 describes an asbestos based material where lignin is added to reduce the need for phenolic resin.
The Seitz et al. U.S. Pat. No. 6,524,681 describes a patterned surface friction material.
The Kaminski et al. U.S. Pat. Nos. 5,889,082 and 5,919,837 describe a friction material comprising a blend of fibers and other friction material components.
The Bartram U.S. Pat. No. 4,197,223 and UK Patent No. 1,604,827 describe mixtures of inorganic and organic fibers such as glass fibers, mineral wools, alumna-silicate fibers, wood pulp, jute, sisal and cotton linters, along with fillers.
The Bortz U.S. Pat. No. 5,989,735 describes fibers carded or woven into a web.
The Ikuta U.S. Pat. No. 5,290,627 describes a friction material having wood pulp and ramie fibers and includes about 10 to about 30% filler.
The Chuluda U.S. Pat. No. 4,256,801 describes a friction material having cellulose, carbon fibers and flame resistant organic fibers (described as novoloid-crosslinked phenolic fibers).
The Royer U.S. Pat. No. 4,743,634 the use of cotton, jute, sisal, wool with friction modifying materials in a friction material.
The present invention is also distinct from patents owned by the assignee herein, BorgWarner Inc., for friction materials. In particular, U.S. Pat. No. 5,998,307 relates to a friction material having a base impregnated with a curable resin where a porous primarily layer comprises at least one fibrous material and a secondary layer of carbon particles covering at least about 3 to about 90% of the surface of the primary layer.
The U.S. Pat. No. 5,858,883 relates to a base material having a primary layer of less fibrillated aramid fibers, synthetic graphite, and a filler, and a secondary layer of carbon particles on the surface of the primary layer.
The U.S. Pat. No. 5,856,244 relates to a friction material comprising a base impregnated with a curable resin where the primary layer comprises less fibrillated aramid fibers, synthetic graphite and filler; and a secondary layer of carbon particles and a retention aid.
The U.S. Pat. No. 5,958,507 relates to a process for producing the friction material where at least one surface of the fibrous material, which comprises less fibrillated aramid fibers, is coated with carbon particles and a retention aid, impregnating with a phenolic or modified phenolic resin, and curing.
The U.S. Pat. No. 6,001,750 relates to a friction material comprising a fibrous base material impregnated with a curable resin where the fibrous base material has a porous primary layer of less fibrillated aramid fibers, carbon particles, carbon fibers, filler material, phenolic novoloid fibers, and optionally, cotton fibers, and a secondary layer of carbon particles which cover the surface at about 3 to about 90% of the surface.
In addition, various base materials are described in commonly owned BorgWarner Inc. U.S. Pat. Nos. 5,753,356 and 5,707,905 which describe base materials comprising less fibrillated aramid fibers, synthetic graphite and filler.
Another commonly owned patent, U.S. Pat. No. 6,130,176 relates to fibrous base materials comprising less fibrillated aramid fibers, carbon fibers, carbon particles and filler.
The U.S. Pat. No. 5,676,577 relates to a friction material having a high amount of filler material in a fibrous base material.
Yet another commonly owned patent, U.S. Pat. No. 6,630,416 relates to a friction material having a porous primary layer and a secondary layer of silica particles covering about 3 to about 90% of the surface of the primary layer.
In still other commonly owned patent applications, U.S. Ser. No. 09/233,318 filed Aug. 30, 2002, Ser. No. 09/234,976 filed Sep. 4, 2002, Ser. No. 09/218,019 filed Aug. 13, 2002, Ser. No. 10/666,090 filed Sep. 19, 2003, Ser. No. 10/678,720 filed Oct. 3, 2003, Ser. No. 10/678,598 filed Oct. 3, 2003, Ser. No. 10/678,725 filed Oct. 3, 2003, Ser. No. 10/678,728 filed Oct. 3, 2003 and Ser. No. 10/678,599, filed Oct. 3, 2003, all currently pending, describe other friction materials.
In order for friction materials to be useful in “wet” applications, the friction material must have a wide variety of acceptable characteristics. The friction material must be resilient or elastic yet resistant to compression set, abrasion and stress; have high heat resistance and be able to dissipate heat quickly; and, have long lasting, stable and consistent frictional performance. If any of these characteristics are not met, optimum performance of the friction material is not achieved.
It is also important that a suitable impregnating resin be used with the fibrous base material in order to form a high-energy application friction material. The friction material must have good shear strength both when saturated with the wet resin during impregnation and when saturated with brake fluid or transmission oil during use.
It is also important, under certain applications, that the friction material have high porosity such that there is a high fluid permeation capacity during use. Thus, it is important that the friction material not only be porous, it must also be compressible. The fluids permeated into the friction material must be capable of being squeezed or released from the friction material quickly under the pressures applied during operation of the brake or transmission, yet the friction material must not collapse. It is also important that the friction material have high thermal conductivity to also help rapidly dissipate the heat generated during operation of the brake or transmission.
The instant friction materials differ from those taught by the above references, particularly since, as far as is known, there is no disclosure of friction material for use in transmission systems which includes a fibrous base material that only contains fibrous material and no filler material. Further, as far as is known, there is no disclosure of a friction material consisting of a fibrous material having dispersed therein a permeating or penetrating of friction modifying particles. Further, until the present invention, there has been no disclosure of such a fiber content only fibrous base material having a permeating or penetrating gradient of friction modifying particles that comprise, for example, symmetrical shaped silica materials. These combinations of ingredients, as described in the embodiments herein, have been found to provide unexpectedly superior results with respect to hot spot resistance, coefficient of friction stability, and low thickness losses.
Accordingly, it is an object of the present invention to provide an improved friction material with reliable and improved properties compared to those of the prior art.
A further object of this invention is to provide friction materials with improved coefficient of friction characteristics, “compression modulus”, “hot spot” resistance, high heat resistance, high friction stability and durability, porosity, strength, and elasticity.
As a result of extensive research and, in view of the need for a better friction material, a friction material with improved characteristics has been developed by the inventors herein. The present wet friction material is useful in “wet” applications where the friction material is “wetted” or impregnated with a liquid such as brake fluid or automatic transmission fluid during use. During use of the “wet” friction material, the fluid is ultimately squeezed from or is impregnating the friction material. Wet friction materials differ greatly, both in their compositions and physical characteristics from “dry” friction materials.